1. Field of the Invention
The present invention relate generally to a data communication system having a network in which relay units connect a central facility with a plurality of terminal units. The terminal units and the relay units carry out data communications in accordance with internal programs contained in the respective units. The present invention relates more particularly to a communication control apparatus used in the data communication system.
2. Description of the Related Art
In a data communication system having a network composed of a central facility, a plurality of relay units, and a plurality of terminal units, the central facility governs all aspects of the data communications including data transmissions and detection of the operating conditions of the respective relay units and the terminal units. The relay units serve as a backup to the central facility and prevents the entire system from breaking down upon failure of the central facility. The relay units transmit data locally to reduce transmission charges. The relay units also ease the load on the central facility.
Conventionally, a predetermined program stored in each relay unit governs transmission of data from the relay unit to its subordinate terminal units or, when superior and inferior relay units are provided in separate levels, from the relay units in one level to the relay units in another level.
Transmission is performed between the terminal units and relay units in an order predetermined by their internal programs. The internal programs are designed to perform transmission efficiently presuming that conditions for transmitting are optimum, that is, when transmission is not impeded in any way.
The internal programs are designed to determine whether the subject unit can communicate with a certain remote unit by attempting to communicate with that remote unit. If the attempt ends in failure, the programs are designed either to again attempt to communicate with the same remote unit or to attempt to communicate with another remote unit. I. other words, there is always a possibility that attempts to communicate with a remote unit will be unsuccessful, but whether communication with a certain remote unit is possible or not can not be ascertained until an attempt is made to communicate. This can result in many superfluous attempts to transmit when the subject remote unit is in a non-communicable condition. This delays the start of data transmission.
Sometimes the central facility is adapted to collect and check the logs sent from the relay units in order to determine whether communication with any of the relay units is being impeded. H/wever, the central facility can collect logs from relay units only during certain time periods. Therefore, a malfunctioning until will not be discovered until a time period arrives when the logs can be checked.